[PATCH v7 0/7] Add support for O_MAYEXEC

Mickaël Salaün
2020-07-23T17:13:06Z

Hi,

This seventh patch series do not set __FMODE_EXEC for the sake of
simplicity. A notification feature could be added later if needed. The
handling of all file types is now well defined and tested: by default,
when opening a path, access to a directory is denied (with EISDIR),
access to a regular file depends on the sysctl policy, and access to
other file types (i.e. fifo, device, socket) is denied if there is any
enforced policy. There is new tests covering all these cases (cf.
test_file_types() ).

As requested by Mimi Zohar, I completed the series with one of her
patches for IMA. I also picked Kees Cook's patches to consolidate exec
permission checking into do_filp_open()'s flow.

# Goal of O_MAYEXEC

The goal of this patch series is to enable to control script execution
with interpreters help. A new O_MAYEXEC flag, usable through
openat2(2), is added to enable userspace script interpreters to delegate
to the kernel (and thus the system security policy) the permission to
interpret/execute scripts or other files containing what can be seen as
commands.

A simple system-wide security policy can be enforced by the system
administrator through a sysctl configuration consistent with the mount
points or the file access rights. The documentation patch explains the
prerequisites.

Furthermore, the security policy can also be delegated to an LSM, either
a MAC system or an integrity system. For instance, the new kernel
MAY_OPENEXEC flag closes a major IMA measurement/appraisal interpreter
integrity gap by bringing the ability to check the use of scripts [1].
Other uses are expected, such as for magic-links [2], SGX integration
[3], bpffs [4] or IPE [5].

# Prerequisite of its use

Userspace needs to adapt to take advantage of this new feature. For
example, the PEP 578 [6] (Runtime Audit Hooks) enables Python 3.8 to be
extended with policy enforcement points related to code interpretation,
which can be used to align with the PowerShell audit features.
Additional Python security improvements (e.g. a limited interpreter
withou -c, stdin piping of code) are on their way [7].

# Examples

The initial idea comes from CLIP OS 4 and the original implementation
has been used for more than 12 years:
https://github.com/clipos-archive/clipos4_doc
Chrome OS has a similar approach:
https://chromium.googlesource.com/chromiumos/docs/+/master/security/noexec_shell_scripts.md

Userland patches can be found here:
https://github.com/clipos-archive/clipos4_portage-overlay/search?q=O_MAYEXEC
Actually, there is more than the O_MAYEXEC changes (which matches this search)
e.g., to prevent Python interactive execution. There are patches for
Bash, Wine, Java (Icedtea), Busybox's ash, Perl and Python. There are
also some related patches which do not directly rely on O_MAYEXEC but
which restrict the use of browser plugins and extensions, which may be
seen as scripts too:
https://github.com/clipos-archive/clipos4_portage-overlay/tree/master/www-client

An introduction to O_MAYEXEC was given at the Linux Security Summit
Europe 2018 - Linux Kernel Security Contributions by ANSSI:
https://www.youtube.com/watch?v=chNjCRtPKQY&t=17m15s
The "write xor execute" principle was explained at Kernel Recipes 2018 -
CLIP OS: a defense-in-depth OS:
https://www.youtube.com/watch?v=PjRE0uBtkHU&t=11m14s
See also an overview article: https://lwn.net/Articles/820000/

This patch series can be applied on top of v5.8-rc5 . This can be tested
with CONFIG_SYSCTL. I would really appreciate constructive comments on
this patch series.

Previous version:
https://lore.kernel.org/lkml/20200505153156.925111-1-mic@digikod.net/

[1] https://lore.kernel.org/lkml/1544647356.4028.105.camel@linux.ibm.com/
[2] https://lore.kernel.org/lkml/20190904201933.10736-6-cyphar@cyphar.com/
[3] https://lore.kernel.org/lkml/CALCETrVovr8XNZSroey7pHF46O=kj_c5D9K8h=z2T_cNrpvMig@mail.gmail.com/
[4] https://lore.kernel.org/lkml/CALCETrVeZ0eufFXwfhtaG_j+AdvbzEWE0M3wjXMWVEO7pj+xkw@mail.gmail.com/
[5] https://lore.kernel.org/lkml/20200406221439.1469862-12-deven.desai@linux.microsoft.com/
[6] https://www.python.org/dev/peps/pep-0578/
[7] https://lore.kernel.org/lkml/0c70debd-e79e-d514-06c6-4cd1e021fa8b@python.org/

Regards,

Kees Cook (3):
exec: Change uselib(2) IS_SREG() failure to EACCES
exec: Move S_ISREG() check earlier
exec: Move path_noexec() check earlier

Mickaël Salaün (3):
fs: Introduce O_MAYEXEC flag for openat2(2)
fs,doc: Enable to enforce noexec mounts or file exec through O_MAYEXEC
selftest/openat2: Add tests for O_MAYEXEC enforcing

Mimi Zohar (1):
ima: add policy support for the new file open MAY_OPENEXEC flag

Documentation/ABI/testing/ima_policy | 2 +-
Documentation/admin-guide/sysctl/fs.rst | 49 +++
fs/exec.c | 23 +-
fs/fcntl.c | 2 +-
fs/namei.c | 36 +-
fs/open.c | 12 +-
include/linux/fcntl.h | 2 +-
include/linux/fs.h | 3 +
include/uapi/asm-generic/fcntl.h | 7 +
kernel/sysctl.c | 12 +-
security/integrity/ima/ima_main.c | 3 +-
security/integrity/ima/ima_policy.c | 15 +-
tools/testing/selftests/kselftest_harness.h | 3 +
tools/testing/selftests/openat2/Makefile | 3 +-
tools/testing/selftests/openat2/config | 1 +
tools/testing/selftests/openat2/helpers.h | 1 +
.../testing/selftests/openat2/omayexec_test.c | 325 ++++++++++++++++++
17 files changed, 470 insertions(+), 29 deletions(-)
create mode 100644 tools/testing/selftests/openat2/config
create mode 100644 tools/testing/selftests/openat2/omayexec_test.c

--
2.27.0